1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, to grey rods having a required reactivity worth for reactivity control of the reactor, such as during and following a period of lower power demand.
2. Description of the Prior Art
In a typical nuclear reactor, such as a pressurized water reactor (hereinafter PWR), the reactor core includes a large number of fuel assemblies. Each fuel assembly is composed of a plurality of elongated fuel rods transversely spaced apart from one another. The fuel rods, each containing fissile material, generate a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant (usually water) is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
One type of PWR fuel assembly, to which the present invention is particularly suited, is known as a "17.times.17" fuel assembly design. In this type of design, the fuel assembly includes a square lattice with 17 fuel rods along each side. This fuel assembly has 264 fuel rods, 24 guide thimbles (for control rods or grey rods discussed below) and one instrument thimble. The outer diameters of the rods are usually on the order of 0.4". See, e.g., co-assigned U.S. Pat. NO. 4,642,216, issued to ORR et al. (hereinafter the "'216 patent") for a further discussion of this type of fuel assembly.
Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, which, in turn, is determined by the neutron flux in the core, control of heat generation at reactor startup, during its operation and at shutdown is achieved by varying the neutron flux. Generally, this has been done by absorbing excess neutrons using clusters of control rods in combination with a soluble neutron absorber. Initially, the level of neutron flux and thus the heat output of the core is regulated by the movement of the control rods into and from the guide thimbles.
The ability of a control rod to absorb neutrons is measured by its relative "reactivity worth." The reactivity worth of a rod can be determined by calculations well known in the art. The basis for calculating the relative value of total and individual rod worths can be an all uranium core or an assumed mixed oxide and uranium core having uranium fuel rods.
Hafnium, silver-indium cadmium, boron carbide, and other materials are known to be strong absorbing or high worth materials. These materials are also termed black absorbers because they are relatively opaque to neutrons. In contrast, stainless steel, zirconium, INCONEL (The trademark for a nickel-based alloy containing 16% chromium and 7% iron and characterized by marked resistance to aqueous corrosion and by high temperature oxidation resistance; also known as Alloy 600) and other materials are known as weak, have relatively low worths and are generally referred to as grey absorbers.
Knowledge of individual rod and rod cluster reactivity worths is of vital importance in controlling the core, as well as determining necessary concentrations of soluble neutron absorbers and additionally in providing fluid moderator, flow rate, density and composition requirements for the reactor.
One common structure adapted for control rods is described in co-assigned U.S. Pat. No. 4,326,919, issued to HILL. This control rod is in the form of an elongated metallic cladding tube having a strong neutron absorbing material disposed within the tube and plugs at opposite ends thereof for sealing the absorber material within the tube. The neutron absorbing material is in the form of a stack of closely packed, high worth ceramic or metallic annular pellets which only partially fill the tube, leaving a void, space or axial gap between the top of the pellets and the upper end plug in defining a plenum chamber for receiving gases which are generated during the control operation. Pellets are used instead of a solid rod to increase the flexibility of the rods and minimize drag during withdrawal and insertion.
Control rods affect reactivity by changing direct neutron absorption and are used for what is known as fast reactivity control. On the other hand, slower, longer term reactivity control is usually carried out by the soluble neutron absorbers and by grey rods which are of low worth relative to the control rods. Grey rods have structures almost identical to the control rods described above, except for the cladding filler. See, e.g., co-assigned U.S. Pat. No. 4,681,728, issued to VERONESI et al. Typically, grey rods have a relatively weak absorber material cladding, e.g. stainless steel, and a relatively weak absorber material cladding filler, e.g. zirconium pellets.
More particularly, the soluble neutron absorbers, such as boric acid, are uniformly distributed in solution throughout the core coolant, leading to more uniform power distribution and fuel depletion than control rods. The concentration of soluble boron is normally decreased with core age to compensate for fuel depletion and fission product buildup.
The buildup of fission products, such as Xenon-135 (hereinafter xenon), reduces reactivity by parasitically absorbing neutrons, thereby decreasing thermal utilization. The xenon is removed by neutron absorption or by decay. Upon a reduction in core power, such as during "load follow maneuvers," fewer thermal neutrons are available to remove the xenon and therefore the concentration of xenon in the core increases. Load follow maneuvers refers to any reactor power changes which are required because of changes in electrical demand. A typical maneuver is a daily load follow, in which the reactor is reduced to a low power value (normally 50 percent) for 6 or 8 hours during the night when electrical demand is at a minimum.
The increase in xenon concentration is usually compensated for by either decreasing the concentration of soluble boron dissolved in the core coolant or by withdrawing the control rods from the core. However, both of these methods have drawbacks. Changing the boron concentration requires the processing of coolant which is difficult and expensive, and therefore not desired by the electrical utility, especially towards the end of core life. Removal of control rods means that the core's return to power capability is reduced. A potential solution to this problem is to use the low worth grey rods in the core at full power. These grey rods are available for removal at reduced power to compensate for the xenon buildup.
As described in co-assigned U.S. Pat. No. 4,707,329, issued to FREEMAN, the drawback of this procedure is that moving these banks of grey rods changes the ever critical axial offset of the core and increases peaking factors. Also, because these low worth grey rod banks are in the core at power, shutdown margins can be affected. This patent suggests as a solution using, instead of grey rods, the full insertion into the core of a control rod whose worth can be changed uniformly in the axial direction during power operation to provide xenon compensation. The control rod is composed of an elongated inner cylindrical member and an elongated outer cylindrical member surrounding the inner member. Each of the elongated members is composed of alternating equal height high worth hafnium and low worth zirconium regions. The inner and outer members are axially movable relative to each other to adjust the degree to which the high and low worth regions of the respective members overlap and thereby change the overall worth of the rod.
This patented control rod approach, however, is complicated due to requiring an elaborate mechanical, moving structure which is difficult to construct and operate, is relatively expensive, and may fail to fully compensate for the buildup of fission products.
The above-cited '216 patent describes a type of grey rod for use with a 17.times.17 fuel assembly, wherein 12 of the 24 strong absorber rods are replaced by 12 stainless steel rods to improve core operations. The disadvantage with this grey rod design is that the combination of strong absorber rods and weak (low worth) rods in the same cluster do not provide for homogeneous absorptions and can result in power peaking penalties.
Further, it is known that the use of low worth grey rods can reduce the coolant processing requirements for a reactor from many thousands of gallons per day to relatively insignificant amounts. A related, important goal is to operate in what is known as a zero boron change load follow (hereinafter ZBCLF) mode, which requires no soluble boron adjustments during load follow maneuvers. Unfortunately, conventional, low worth grey rods are incapable of allowing ZBCLF at all power conditions because their cumulative worth is deficient.
One potential modification to the grey rods to increase their relative worths, at least when the diameter of the grey rods is rather large, e.g., 0.8 to 1.0" O.D., is to vary the size of the central hole in the annular pellets. That is, by decreasing the size of the hole, it is possible to increase the volume fraction of absorber material, thus increasing the reactivity worth. As a result, the number of grey rods in a typical design might actually be reduced from the normal compliment because of the increased reactivity worth.
Reducing the number of grey rods is desired for two reasons: first, it simplifies the mechanical design of the upper internals and reactor vessel head/integrated heat package; and second, there is a significant reduction in capital cost due to elimination of some of the assembly, driveline mechanism, cabling, etc., costs associated with use of the larger number of rods.
However, this alternative is not adaptable for all fuel assemblies. For example, it is not applicable to a standard 17.times.17 reactor, because the outer diameters of the rods used therein are too small (about 0.381 inch). In fact, even with approximately 100% volume fraction of stainless steel or INCONEL (i.e., 0.341 inch pellets inside a 0.381 inch O.D./0.344 inch I.D. cladding) the reactivity worth of the individual grey rods in a particular application would probably be insufficient to achieve ZBCLF: additional grey rods of the same (stainless or INCONEL) design would probably be required. However, the addition of more grey rods would increase the mechanical complexity of the reactor vessel internals/head area/integrated head package/etc. and would significantly increase the capital cost.
Notwithstanding a theoretical desire to reach ZBCLF by increasing grey rod worth, a designer must be able to retain the desired stiffness criteria for grey rods, as well as prevent the weight of the grey rods from increasing, which would increase driveline mechanism requirements.
Thus, merely increasing significantly the cladding thickness to increase worth would not only undesirably increase stiffness, but would also be difficult and expensive to manufacture.
Moreover, using a solid grey rod of a relatively weak neutron absorber material would be less expensive than the thick walled tube suggested immediately above, but would further frustrate the stiffness criterion. A solid rod would also violate the weight criterion unless the O.D. of the rod were reduced. However, using a small rod might lead to vibration and wear problems in both the core thimbles and in the upper guide structures since these components were designed to accommodate rods of a certain outer diameter. In light of the above, a grey rod design which has the required worth to achieve ZBCLF, is adaptable to a "17.times.17" fuel assembly, is economically manufactured, and can be incorporated without complication of the remaining reactor structure or operation, is still desired.